Biomedical Engineering Reference
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Fig. 19 From left to right (a): free ligand 18,(b) 18 +F ,(c) 18 +Cl ,(d) 18 +Br ,
(e) 18 +I ,(f) 18 + HSO 4 ,(g) 18 +H 2 PO 4 ,(h) 18 +F +Cl +Br +I (10 equiv. of
tetrabutylammonium salts for each anion were used)
OO -
NOE
N +
O
N + O
Bu 4 N +
HF 2 - ยท 2Bu 4 N +
H
H
O
O
O
O
N
O
O
N
N
N
O
H
H
O
O
N
O
H
O
O
N
H
N +
O
N +
O
-
O
Fig. 20 1:1 complex formed between 18 and F ( left ); deprotonated specie responsible for the
color ( right )
complex anions at the thiourea moiety, as well as cations such as Ni 2+
at the
bipyridine moiety. This fact can confer allosteric properties to these ligands.
Optical properties of bipyridine systems are strongly dependent on the solvent
[ 43 , 44 ]. Thus, compound 24, for example, shows a maximum of absorption at
236 nm in acetonitrile and at 258 nm in dimethyl sulfoxide (DMSO). In both
solvents, a shoulder at 280 nm is observed, which corresponds to the phenylamido
chromophore. In order to clarify this compound's solvatochromic behavior, a set of
solvents was used. It was concluded that solvents with strong hydrogen-bond
acceptor (HBA) properties [DMSO, tetrahydrofuran (THF) and ethyl acetate]
induce a bathochromic shift, whereas those with strong hydrogen-bond donor
(HBD) properties (methanol and ethanol) give rise to a clear hypsochromic effect
of the UV absorption maxima. Differences in the absorption maximum can relate to
the presence of different conformations in solution, depending on the solvent's
characteristics. Thus, ligands 22 and 23 appear in DMSO solutions as s-trans
conformers, which is stabilized through the formation of hydrogen bonds between
the NH of the thiourea group attached to one ring with the pyridine nitrogen in the
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